. An Electrical Measuring Machine for Engineering Gauges and Other Bodies . nother invar rod L is screwed to K. Onthe end of L is screwed a microscope slide. The right screw-end cap has asmall lens mounted on it. Set up the slide A to reflect sodium light, view withthe microscope Mi the Newtons rings formed between S and the lens surface. The micrometer scale is read for every 50 rings passing the cross-wires(the length traversed is thus 14*73 /a, taking X = 0*589 /u). The interfering surfaces are never more than 200 half wave-lengthsapart—so that the rings are quite clear always. Also, as the
. An Electrical Measuring Machine for Engineering Gauges and Other Bodies . nother invar rod L is screwed to K. Onthe end of L is screwed a microscope slide. The right screw-end cap has asmall lens mounted on it. Set up the slide A to reflect sodium light, view withthe microscope Mi the Newtons rings formed between S and the lens surface. The micrometer scale is read for every 50 rings passing the cross-wires(the length traversed is thus 14*73 /a, taking X = 0*589 /u). The interfering surfaces are never more than 200 half wave-lengthsapart—so that the rings are quite clear always. Also, as the surfaces are sonear, the number 0*589 fi may be taken without considering the exact valuesfor the Di and D2 sodium lines. As soon as 200 bands have been read, the left screw is advanced to bringthe interfering surfaces into contact again. Then proceed as before. Curve II is the result, in which the periodic error of Curve I has I and II refer to position 25—26 mm. on the right screw. 354 Dr. P, E. Shaw. An Electrical Measuring [Dec. 1* Fig. 9. •510. Microns 1513 Curves III and IV refer to 15—16 mm. and 25—26 mm. respectively onthe left screw. The scale of abscissae is an open one, so that plotting in thisway is a very severe test of a screw. Each millimetre requires about 80 readings, each of 50 bands. Where a 1905.] Machine for Engineering Gauges and other Bodies. 355 curve looks irregular, readings are repeated, but in nearly all cases thereadings were found correct to 0*05 //,. The millimetre of the screw corre-sponding to Curve II is more regular than any other tested, the length of runof the screw for 50 bands lying in all the 80 cases between the narrow limits14-72 n to 14-90 p. These calibration readings are very exact, and can almost alwTays berepeated to \/20 = 0*03 micron. Higher accuracy could be attained byspecial precautions, but gauge work does not call for it. Some precautions taken in this interference work are:—(1) Leave theapparatu
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